All mammals begin as two cells — sperm and egg — that combine into a single cell. This single cell will divide exponentially into specialized cells making up various organs and systems — all the tissues of a new organism.
Simply put, a stem cell is an immature cell that can become a different cell, or perhaps become one of many different cells.
Most stem cells also can renew themselves — divide — indefinitely. These two characteristics are what present a new pathway to repairing damage to the human body caused by trauma, degeneration and disease.